Question

I am using Arduino Uno to power a Nokia 6100 LCD. In my program, i take RGB 8 bit input which can be converted to 24 bit Hex using any available online conversion tools, that conversion i can take care of. However, the LCD library i am using only allows 12 bit Hex. how can i get the LCD to accept 24 bit Hex values instead of 12 and get the right colour on the screen. Or is there a way to change from 24 bit Hex to 12 bit hex in this case?

Thanks,
Faiz

Answer 1

Perhaps the LCD only understands 4bit channels? In any case, it sounds like that’s the limit of the library 🙂

To convert to a 12-bit value (3 channels x 4 bits/channel) from a 24-bit value (3 channels x 8 bits/channel), just scale down each channel by a factor of 2⁴ — that is, divide each 8-bit channel value [0-255] by 16 to obtain the approximate value in a 4-bit channel [0-15].

Now, consider this: “dividing by 16” and “shifting right by 4” (non sign-extending) is effectively the same for unsigned 2’s complement integers. That is, the bottom 4 bits are just “thrown out”.

Imagine this 24-bit value, in bits (padded in 32-bit integer):

00000000RRRRrrrrGGGGggggBBBBbbbb 

And this is the target value (padded in 16-bit integer):

0000RRRRGGGGBBBB  // just get rid of the "small letter" bits :)

And note that this can be obtained with a series of bit-wise operations:

r12 = (hex24 >> 20) & 0xF
g12 = (hex24 >> 12) & 0xF
b12 = (hex24 >> 4) & 0xF
hex12 = (r12 << 8) | (g12 << 4) | b12

Happy coding.